Hydrolysis of monomolecular layers of synthetic sphingomyelins by sphingomyelinase of Staphylococcus aureus.

The enzymic hydrolysis of three synthetic sphingomyelins, spread as monomolecular films at the air/water interface by purified Staphylococcus aureus sphingomyelinase was studied. Each of the three sphinomyelins (DL-erythro-N-palmitoyl-, -N-stearoyl- and -N-lignoceryl-sphingosylphosphocholine) has an optimal activity-dependent surface pressure or concentration curve. The optimal surface pressure as well as the optimal surface density for hydrolysis was different for each of the three substrates. This optimum coincides with the liquid-condensed/liquid-expanded phase transition for each of the sphingomyelins. At initial surface pressures (pi 0) below the optimum, reaction rates are controlled mainly by surface density of the substrate; above the optimal pi 0, reaction rates decrease with increasing surface pressure. The difference between the three synthetic sphingomyelins are explained by the variation in the degree of asymmetry between their two paraffinic chains.